1. Field of the Invention
This invention relates to the field of disk drives, and more specifically to a method and apparatus for fastening a head-suspension assembly to a rigid actuator arm for a hard disk drive.
2. Description of the Related Art
The data storage capacity of a hard disk drive is determined by its volumetric recording bit density, i.e., the number of recording bits per unit volume that the disk drive can use. Using one or more recording disks in a hard disk drive, the total available bit storage capacity in a hard disk drive is a function of the following parameters: the number of recording bits per recording track on a surface of a recording disk, the number of recording tracks on the surface of the recording disk, the number of recording surfaces for each recording disk, and the number of recording disks contained in the hard disk drive. To increase the data storage capacity of a hard disk drive, one may increase any or all of the above parameters. There is, accordingly, a clearly felt need to more effectively use the space within a disk-drive enclosure as well as a need to increase the durability of the drive through increased shock protection. More efficient use of space can increase the number of recording disks within the hard disk drive (which greatly increases the amount of data that can be stored) and/or provide additional space for other internal components.
A head-suspension assembly for a disk drive typically includes a magnetic transducer, called the read/write head, capable of detecting and/or changing the magnetic transitions on the magnetic recording surface of a magnetic recording disk. The read/write head is typically fixed to a gimbal assembly that enables the read/write head to fly above, or be in full or partial contact with, a moving magnetic recording surface. The gimbal assembly is fastened to a load beam, and may be formed out of load beam material. The load beam is typically made of thin, somewhat flexible material, usually metal. The head-suspension assembly is typically fastened to an actuator arm. The actuator arm is moved by the control electronics of the disk drive to position the read/write head over different portions (tracks) of the magnetic recording surface. Moving the read/write head from one track on the magnetic recording surface to another track on the same magnetic recording surface, so that the read/write head can read the data on the new track requires a time interval called access time. Another factor affecting drive performance is rotational latency time, which is the time it takes the disk to rotate from the present location on the desired track to the location of the desired data on the same track.
One constraint on developers of hard disk drives is the "form factor" (the defined physical dimensions) imposed on a hard disk drive to be competitive within the market-place. In particular, each form factor defines a height that drives must match so that the drive fits correctly into standard openings in devices using the drives, such as personal computers and other electronic devices. Increasing the number of magnetic recording disks within a given form factor reduces the spacing between adjacent magnetic recording disks. The actuator arms and associated suspensions and magnetic read/write heads operate within the space between adjacent disks, and on the outfacing surfaces above and below the "top" and "bottom" disks respectively. The assembly and operation of the read/write heads is more difficult because the reduction of the space for the read/write headsuspensions places the head-suspension materials in close proximity to the moving disk surface.
The typical read/write head-suspension assembly includes a load beam that is welded to a nut plate. The nut plate provides a stiff weldment for the suspension and the means for attaching the suspension to the disk-drive actuator arm. One commonly used method for attaching the headsuspension assembly (also called the "Head-Gimbal Assembly" (HGA), particularly when a gimbal is included for the head assembly) to the disk drive actuator arm is by swaging. In the swaging process, a spherical swage ball is passed through a tube that provides the attachment feature of the nut plate. The material of the nut plate tube (also called the "ferrule" or "boss" of the nut plate) and the actuator arm is displaced to accommodate the swage ball being forced through the nut plate tube (the diameter of the swage ball is larger than the original inner diameter of the nut plate tube). It is important that the flange portion of the nut plate remain flat and stable during the swaging process so that the suspension performance is not compromised by undesirable deformations of the nut plate. Therefore, the nut plate flange has typically been manufactured so that the nut plate flange provides a stiff mounting for the suspension. This stiffness is provided by providing a relatively thick nut plate flange, typically three (3) to four (4) times the thickness of the suspension material. Prior-art FIG. 1 is a side view of the portion of a head-arm assembly 100 showing the head-suspension material 120 fastened between a nut plate 130 and an actuator arm 110. Portions of the complete actuator arm 110 and complete head-suspension 120 are not shown to simplify the drawing, but are well known to those skilled in the art. The attachment method is typically the conventional swaging process exemplified in FIG. 2. The swaging process necessitates a relatively thick nut plate flange 133, that reduces the nut plate-to-disk clearance 142 (shown in FIG. 1) to undesirably low values. FIG. 2 shows the nut plate slipped into a swaging hole 111 in arm 110 and shows the swage ball 150 immediately before it is forced through the nut plate tube 131. The interference fit of the swage ball 150 with the nut plate tube 131 forces the material of the nut plate tube 131 radially outward into the actuator arm 110, fixing the nut plate 130 solidly to the actuator arm 110 at surface 132.
The present practice is to place the nut plate flange material on the side of the suspension material that is closest to the disk surface. By doing so, the clearance to the disk surface is reduced still further. This is not desired because it requires tight tolerance control to assure no contact between the nut plate flange and the disk surface during the head merge operation wherein the actuator components are first introduced to the disk stack at manufacture, and later during operation of the disk drive. Tight tolerances require more intricate tooling and fixtures, as well as increased piece part cost. It is also important to avoid contact between the nut plate and suspension components and the disk surface during shock loading, both during disk operation and otherwise. Shock-induced contact may cause the localized loss of data at the location where a head, load beam, nut plate, or actuator arm hits the disk surface, or catastrophic failure of the drive in the form of a head crash.
Others have approached the shock-event situation a bit differently in the portable market. Some actually drive the actuator clear off of the disk 140 when parking the heads for shutdown. If the arms and suspensions are not over the disk, then they do not have to worry about disk-to-head or disk-to-suspension contacts during non-operational shock events. This approach has several significant disadvantages:
(1) A ramp tower is needed to keep the suspensions spaced apart when they are unloaded from the disk; PA1 (2) It requires more space in the drive: the ramp tower space as well as the additional rotation angle space for the actuator to unload the heads; PA1 (3) It uses energy to park and unpark the heads from the tower; PA1 (4) Loading and unloading heads can introduce instability in the flight of the heads, leading to greater risk of drive failure; and PA1 (5) Operational shock protection is not improved by this strategy.
Another approach is to thin the nutplate material. One embodiment initially used this approach by reducing the nutplate thickness to 0.0098" (0.25 mm) from 0.0118" (0.30 mm). This only gains 0.002" (0.05 mm). Suspension suppliers and head stack suppliers are reluctant to thin the nutplate further, because of concern for warpage of the nutplate during the standard swage-attaching operation. This warpage results when the swage ball is forced through the ferrule (or boss) on the nutplate. Swaging is a harsh event. Nutplate stiffness limits warpage only if the nutplate is thick enough. Practitioners believe that the 0.0098" (0.25 mm) is minimal but that substantial warpage would likely result with thinner nut plates. Warpage is a problem because it affects the bend area of the suspension. This area is carefully designed to achieve acceptable resonance characteristics and to achieve adequate loading of the slider against the disk. This loading (called the "gram load") must be well controlled to achieve consistent flight behavior of the slider of head suspension 320.
Adhesive fastening is believed to have been used on the suspension-to-load beam or flexureto-actuator arm assembly by Maxtor Corporation on a 5 1/4-inch (133 mm) drive, 8 disks, about a 640-800 MB drive made about 1988-89. The adhesive used was a two-part adhesive, where one side of one piece is coated with adhesive Part A, and the facing surface of the other piece is coated with adhesive Part B. When the two adhesive parts come in contact, they cure in a chemical reaction at room temperature (RT). To set the adhesive, the two pieces are placed in a tool that holds them in contact until the adhesive sets (about 30 minutes at RT). The type of adhesives used by Maxtor Corporation are now considered too unstable for use at the upper operating temperatures of modern disk drives, are also considered as having unacceptable out-gassing for modern disk drives, and also are not stable enough for applications in modern very small drives designed for extremely long lifetimes.
Another problem known in the art of fastening the head-suspension assembly to the actuator arm arises from vibrations. Consider that vibrations are passed from the actuator arm to the head-suspension assembly. The rigid metal-to-metal connection, for example, between the swaged nut-plate tube and the actuator arm, apparently passes vibrations from the actuator arm to the head, which then occasionally introduces head-tracking errors.
Another problem known in the art is the impracticality in reworking the actuator arm-suspension assembly when a problem is detected. Removing the head-suspension assembly from the actuator arm without damage to either the head-suspension assembly or the actuator arm is extremely difficult. Presently, removing a head-suspension assembly from an actuator arm damages the head-suspension, rendering it useless.
Other problems known in the art are the relatively long time needed to assemble the slider/head-to-suspension assembly, and the impracticality in reworking when a problem is detected. Currently the slider suspension and read/write head assembly (together often just called the "head") is bonded to the suspension with an electrically-conductive adhesive, such as a silver-filled two-part epoxy adhesive, and an anaerobic, UV and heat-curing adhesive, such as Loctite 366. The 366 adhesive is used as the fast-bonding material to hold the head and suspension in position, while the filled-epoxy adhesive requires longer heat-cure time to react. The process requires depositing small drops of each adhesive component, followed by UV and heat-cure cycles. A typical process would include a fast-UV cure to tack the assembly, followed by a heat-cure tunnel oven to react the conductive epoxy adhesive. This is a time-consuming process. In addition, if a problem is detected, removing a head from a suspension damages one or both, rendering them useless.
U.S. Pat. No. 4,943,875 and U.S. Pat. No. 4,853,811 disclose methods and means for attaching the head-suspension assembly to the actuator arm through the use of screws or welds. U.S. Pat. No. 4,943,875 disclose the use of a mounting band that encircles the end of the actuator arm and is secured thereto by one of a variety of means, including spring loading, adhesive, solder, or deformable metallic means.